Method of Communication Using Red/yellow Flags
Puzzle 6 of Countdown to 2020 Piece of Cake Pack
Question is: Method of Communication Using Red/yellow Flags
and the answer is: SEMAPHORE
Word Meaning:
SEMAPHORE — Any equipment used for visual signalling by means of flags, lights, or mechanically moving arms, which are used to represent letters of the alphabet, or words.
Definitions from Wiktionary.